In optical systems such as dense wavelength division multiplexing (DWDM) systems, low frequency modulations can be applied to wavelength channels to carry channel wavelength information and other identification information, which improves fiber link management and enables power monitoring. Low frequency modulation based channel monitoring, e.g., tone based channel monitoring, can be used for various applications (e.g., for DWDM systems). However, its applications are limited by the stimulated Raman scattering (SRS) caused crosstalk of modulated optical signals in the fiber links. This crosstalk can substantially distort the low frequency modulations in optical signals and hence reduce channel monitoring efficiency and performance. There is a need to reduce or suppress this SRS crosstalk in the signals.